Needs for liquid crystal displays for the application to such as an LCD television and a personal computer display have expanded. Usually, a liquid crystal display is constituted of a liquid crystal cell formed by sandwiching transparent electrodes, a liquid crystal layer, a color filter and so on with glass plates, and two polarizing plates provided on both surfaces of the liquid crystal cell. The polarizing plates each have a structure in which a polarizer (also referred to as a polarizing membrane, or a polarizing film) is sandwiched with two optical films (also referred to as polarizing plate protective films).
Various kinds of liquid crystal displays having different display modes have been developed so far. For example, there have been known a twisted nematic mode (also referred to as a TN mode), a super-twisted nematic mode (also referred to as an STN mode), an in-plain switching mode (also referred to as a transverse electric field mode or an IPS mode), and a vertical alignment mode (also referred to as a vertical alignment mode or a VA mode).
Specifically, as a display mode for large sized liquid crystal displays such as a large sized monitor or a liquid crystal television, the VA mode which enables a high display speed and an excellent front contrast is becoming a dominant display mode. In a vertical alignment mode liquid crystal display such as a VA mode liquid crystal display, the long axis of a liquid crystal material is vertically aligned against the substrate face when no voltage is applied. Accordingly, a vertical alignment mode liquid crystal display has a feature that, when a liquid crystal display is viewed from a vertical direction against the substrate, a nearly perfect black display is attained, whereby a high contrast is obtained.
However, the vertical alignment mode liquid crystal display has a problem that coloring in a black display due to light leakage, or reduced contrast is observed when the display is viewed from an oblique direction. This problem is ascribed, as one of the possibilities, to the retardation (also referred to as a retardation) in the thickness direction of the liquid crystal layer itself used in the liquid crystal cell. Further, it is also considered to be because the transmission axes of two polarizing plates provided on both surfaces of the liquid crystal cell and arranged in a crossed Nicol condition crosses perpendicularly when the display is viewed from the front direction, however, when the display is viewed from an oblique direction, the apparent crossing angle of the transmission axes deviate from 90°.
In order to over come the problem that coloring in a black display due to light leakage is observed, there have been known a technique in which a uniaxial negative birefringent optical compensation film having retardation in the thickness direction (also referred to as a C plate) is employed to compensate the retardation in the thickness direction of the liquid crystal layer used in the liquid crystal cell (refer to Patent Document 1). Also, there has been known a technique in which a uniaxial positive birefringent optical compensation film (A plate) is employed (refer to Patent Documents 2 and 3). Accordingly, in principle, it is expected that the retardation in a vertical alignment mode liquid crystal display can be compensated by employing such uniaxial optical compensation films in combination. Although the above disclosed optical compensation films are effective to compensate retardation for a specified wavelength of light, the effect is not fully enough for light of a wavelength other than the specified wavelength and, also, there has been a problem that coloring in a black display due to light leakage, or reduced contrast is observed when the display is viewed from an oblique direction.
In order to overcome this problem, it is necessary to provide an optical compensation function not only at a specific wavelength but also in all over the visible light region. Therefore, in order to compensate the apparent shift of the axes of the polarizers, optical compensation may be conducted to light of shorter wavelength and longer wavelength as equally as possible by using an A plate which exhibits a larger in-plane retardation when the wavelength becomes longer, namely, a reverse wavelength dispersion of the in-plane retardation. In general, the retardation in the thickness direction of the liquid crystal layer used in the liquid crystal cell of a VA mode liquid crystal display exhibits no wavelength dependence (namely, the wavelength dispersion of the retardation value is flat) or the retardation in the thickness direction becomes larger when the wavelength becomes shorter (namely, a normal wavelength dispersion). Accordingly, it is desirable to use an optical compensation film which exhibits a larger retardation in the thickness direction when the wavelength is increased, namely, the retardation in the thickness direction exhibits a normal wavelength dispersion (C plate) to compensate the retardation in the thickness direction of the liquid crystal layer. Also, proposed has been a technique to use two uniaxial films to compensate the retardation not only at a specific wavelength, but also in all over the visible light region (for example, refer to Patent Document 4). Namely, in this technique, it is expected that optical compensation in a wide wavelength range can be attained by employing an A plate which is a uniaxial optical compensation film having an in-plane retardation, in which the in-plane retardation exhibits reverse wavelength dispersion, and a C plate which is a uniaxial optical compensation film having a retardation in the thickness direction, in which the retardation in the thickness direction exhibits normal wavelength dispersion. However, it is very difficult to produce an optically uniaxial optical compensation film as described above employing a resin material commonly used for a polarizing plate protective film, such as a cellulose ester film, and it is necessary to use a material having a large photoelastic coefficient to produce a uniaxial film using a resin. Therefore, unevenness in a retardation value tends to occur, and as the results, suppression of light leakage in an oblique direction becomes difficult. A uniaxial film formed from such a material generally exhibits poor adhesion with a polyvinylalcohol resin commonly used as a polarizer, and it tend to be difficult to directly adhere with such a polarizer, and, when it is directly adhered with a polarizer, unevenness in retardation tends to occur, which also results in occurrence of light leakage. Accordingly, a conventionally used polarizing plate protective film has also been necessary between a uniaxial film and a polarizer, which has been a problem because of increase in the thickness of a liquid crystal display and a manufacturing cost. Also, there has been known a technique to form an optical compensation layer by applying a liquid crystal layer on a resin substrate of such as a cellulose ester resin, followed by orienting. However, this technique also results in problems, for example, a thicker liquid crystal display panel and a higher manufacturing cost due to a complicated manufacturing process.
As a countermeasure to these problems, an optical compensation film having a uniform retardation, and which can provide a thinner liquid crystal panel can be easily manufactured as an optical compensation film by employing a biaxial resin film prepared by providing a prescribed retardation function to a resin film which has been commonly used as a polarizing plate protective film (for example, refer to Patent Document 5). However, it has been difficult to obtain a single biaxial resin film which enables sufficient optical compensation of a VA mode liquid crystal display apparatus. Accordingly, there has been proposed a technique to employ two biaxial resin films sandwiching a liquid crystal cell, each biaxial resin film being constituted of a cellulose ester film which also has a function of a polarizing plate protective film and is provided with a prescribed retardation value, to provide a necessary optically compensating function to a VA mode liquid crystal display as well as to reduce the thickness of the liquid crystal display, whereby the manufacturing process becomes simpler (for example, refer to Patent Document 6). However, according to the technique disclosed in Patent Document 6, there has been a problem that the in-plane retardation and the retardation in the thickness direction have similar features in wavelength dispersion, since the optical compensation is carried out by using the in-plane retardation and the retardation in the thickness direction exhibited by each of the two biaxial optical compensation films. Namely, while mutually different wavelength dispersions are necessary to compensate the apparent misalignment of the axes and the retardation in the thickness direction of a liquid crystal cell in all over the visible light region, faced has been a problem that it is difficult to independently control the in-plane retardation value and the retardation value in the thickness direction. For example, when the in-plane retardation is controlled to show reverse wavelength dispersion in order to compensate the apparent misalignment of the axes of the polarizers, the retardation in the thickness direction also shows reverse wavelength dispersion, which is contrary to the desired wavelength dispersion for the for the retardation of the liquid crystal layer of the liquid crystal cell, whereby compensation over a wide wavelength range becomes difficult. Alternatively, when the retardation in the thickness direction is controlled to show flat or normal wavelength dispersion, the in-plane retardation also shows similar wavelength dispersion, whereby compensation of the misalignment of the axes over a wide wavelength range becomes difficult. Thus, it has been difficult to effectively compensate the apparent misalignment of the axes of the polarizers and the retardation in the thickness direction of the liquid crystal layer used in a liquid crystal cell in all over the visible light region. The present inventors have conducted investigation to solve these problems, however, independent control of the in-plane retardation and the retardation in the thickness direction of a biaxial optical compensation films, thus improvement has been desired.